1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that is adaptive for improving display quality.
2. Discussion of the Related Art
A liquid crystal display device controls the light transmittance of a liquid crystal by use of an electric field, thereby displaying a picture.
FIG. 1 is a diagram illustrating a liquid crystal display device of the related art. In FIG. 1, a detailed illustration of a thin film transistor panel 20 is omitted.
In FIG. 1, a liquid crystal display device of the related art includes a thin film transistor panel 20 and a color filter panel 5 which face each other and are bonded together; and liquid crystal molecules 10 filled in a liquid crystal space provided between the two panels 20, 5.
A lower polarizer 1 and an upper polarizer 3 are attached to the outer surfaces of the color filter panel 5 and the thin film transistor panel 20, respectively.
Generally, optical transmission axes of the lower polarizer 1 and upper polarizer 3 of the liquid crystal display device perpendicularly cross each other, and the lower polarizer 1 and the upper polarizer 3 only transmit the lights which are parallel to their own optical transmission axes, respectively.
Although not illustrated, the thin film transistor panel 20 includes gate lines formed on a lower substrate; and data lines which cross the gate lines to define pixel areas. And, in the pixel area are formed a thin film transistor which is a switching device; a pixel electrode connected to the thin film transistor; and a common electrode to form an electric field with the pixel electrode.
If the electric field is formed between the pixel electrode and the common electrode, the liquid crystal molecules 10 arranged between the thin film transistor panel 20 and the color filter panel 5 rotate by dielectric anisotropy. The transmittance of the light transmitted through the pixel area is changed in accordance with the degree of rotation of the liquid crystal molecules 10, thereby realizing the picture.
The color filter panel 5 includes a substrate 17; and a black matrix 15 and a color filter layer 13 which are sequentially formed on the substrate 17.
The black matrix 15 is formed in a matrix pattern on the substrate 17. The black matrix 15 divides the area of the substrate 17 into a plurality of cell areas and prevents optical interference between adjacent cells and external light reflection. The color filter layer 13 is formed to include red, green and blue color filters R, G, B which are formed in the cell area divided by the black matrix 15. The red, green and blue color filters R, G, B transmit red, green and blue lights respectively.
The liquid crystal molecules 10 formed in the liquid crystal display device have refractive index anisotropy and have a distribution state that changes based upon viewing angle directions. Thus, the light transmitted through the liquid crystal molecules 10 has a phase delay value different for each viewing angle direction. If the phase delay value is different in accordance with the viewing angle, brightness and contrast ratio become different for each viewing angle direction, thereby deteriorating the display quality of the liquid crystal display device.
In order to improve the deterioration of brightness and contrast ratio caused by the distribution state of the liquid crystal molecules 10 in accordance with the viewing angle direction, the related art includes first and second compensation layers 11, 19 which compensate the phase delay difference in accordance with the viewing angle.
The first compensation layer 11 is formed to cover the color filter layer 13, and the second compensation layer 19 is formed between the color filter panel 5 and the upper polarizer 3. The first compensation layer 11 is made by hardening a polymerizable liquid crystal, and the second compensation layer 19 is mainly made by drawing a polymer film.
On the other hand, light has dispersion characteristic that light is refracted differently for each wavelength range when being transmitted through a medium. In other words, when the light is transmitted through the medium, the light has a different phase delay value for each wavelength range. Accordingly, the red, green and blue lights transmitted through the lower polarizer 1, the liquid crystal 10, the first compensation layer 11, the color filter layer 13, the second compensation layer 19, the upper polarizer 3 and the like are phase-delayed to have different values from each other, thus a desired color is not displayed in the liquid crystal display device and the display quality of the liquid crystal display is deteriorated. Particularly, the phase delay difference is high by wavelength ranges after the light incident from a light source is transmitted through the first compensation layer 11 being made of polymerizable liquid crystal. This is because the dispersion characteristic of the liquid crystal is basically high.
FIG. 2 illustrates phases of red, green and blue lights on a Poincare sphere.
In conjunction with FIG. 2, to describe the phase delay of the red, green and blue lights more specifically, the red, green, blue lights having the same phase as shown in area ‘X’ when being initially incident from the light source are phase-delayed to have different values from each other as the lights are transmitted through the first compensation layer 11, thereby having different phases as shown in area ‘Y’. At this moment, the reason why the phase delay values of the red, green and blue light are different is because light has a different phase delay value Γ in accordance with a wavelength λ.Γ=2πdΔn/λ  [Mathematical Formula 1]                (Γ: phase delay, d: thickness of medium, Δn: refractive index of medium, λ: wavelength)        
As shown in Mathematic Formula 1, the phase delay value is lower as the wavelength λ is higher, and the phase delay value is higher as the wavelength λ is lower. The wavelengths λ of red R, green G and blue B are 650 nm, 550 nm and 450 nm respectively, thus the red light with the long wavelength λ has the lowest phase delay value among the red, green and blue, and the blue light with the short wavelength λ has the highest phase delay value among the red, green and blue, as shown in area ‘Y’ of FIG. 2.
The red, green and blue lights having phases as shown in area ‘Y’ are phase-delayed with the same principle as described above in Mathematical Formula 1 when being transmitted through the compensation layer 19 and the upper polarizer 3, thereby having more dispersed phases as shown in area ‘Z’.
Further, the optical axis of the upper polarizer 3 of the related art is designed to be in accord with the optical axis of the green wavelength range, thus the red and blue lights having different phases from the green cannot be displayed correctly, thereby deteriorating the display quality of the liquid crystal display device. Hereinafter, the case of displaying a black state, i.e., the case that the optical axis of the light transmitted through the liquid crystal molecules 10 is perpendicular to the optical axis of the upper polarizer 3, is taken as an example to fully explain the display quality deterioration of the liquid crystal display device caused by the phase delay difference for each wavelength range.
When the liquid crystal display device is in a black state, the optical axis of the upper polarizer 3 designed in consideration of the wavelength of green when observing the liquid crystal display device in a viewing angle direction becomes perpendicular to the optical axis of green, thereby shutting off the green light. But, on the other hand, as described above, the red and blue lights having different phase delay values from the green light does not have their optical axes correctly perpendicular to the optical axis of the upper polarizer 3. The red and blue lights not being perpendicular to the upper polarizer 3 can leak out through the upper polarizer 3. If the red and blue light leak out, an unwanted color of purple made by mixing red and blue is displayed on a screen of the liquid crystal display device, thereby deteriorating the display quality of the liquid crystal display device.